In the field of radio communication using high frequency, signals having a particular frequency are extracted from many other signals, thereby separating necessary signals from unnecessary signals. Circuits that serve this function are referred to as a filter and used in many radio communication devices. As the frequency of the signal extracted by the filter becomes higher, the center frequency becomes higher, and the bandwidth also increases. If the bandwidth increases, the filter permits signals in the adjacent channel to pass therethrough, and this causes occurrence of an interference wave. To avoid this, it is necessary that both the center frequency and the bandwidth can be controlled and varied. A filter capable of varying both the center frequency and the bandwidth disclosed in the Patent literature 1 is shown in FIG. 31, and an operation thereof will be described below. Signals at plural frequencies are input, via an input terminal 301 and a transmission line 303, to a band control circuit 305 composed of a direct-current cut capacitor 313 and a varactor diode (a variable capacitor) 314 connected in series. A resonator 304 is connected between the output of the band control circuit 305 and the ground. The resonator 304 is composed of a parallel connection of a resonant coil 307, a resonant capacitor 308, and a series circuit composed of a capacitor 309 and a varactor diode 310. The connection point between the band control circuit 305 and the resonator 304 is connected to an output terminal 302 via a direct-current cut capacitor 306.
To raise the resonance frequency of the resonator 304, that is, the center frequency of the filter, the voltage applied to a frequency control terminal 311 for varying the capacitance of the varactor diode 310 of the resonator 304 is raised, thereby reducing the capacitance of the varactor diode 310. At this time, if the capacitance of the direct-current cut capacitor 313 on the signal input terminal remains unchanged, the bandwidth also increases. To avoid the increase of the bandwidth, the voltage applied to a band control terminal 315 of the varactor diode 314 of the band control circuit 305 is also raised, thereby reducing the capacitance of the varactor diode 314. As a result, the increase of the bandwidth caused by increasing the center frequency of the filter can be prevented. There has been proposed a filter that can vary both the center frequency and the bandwidth to a desired value by varying the coupling capacitance of the resonator.
However, as can be seen from the circuit diagram of FIG. 31, this filter is composed of lumped constant elements, and it is difficult to use the filter in the microwave band used for mobile communication as it is, for example. In addition, this filter varies the resonance frequency by varying the capacitance of the varactor diodes. However, the temperature characteristics of the capacitance of such a device is unstable, so that the reproducibility of the resonance frequency is low. Thus, for example, the applicant has disclosed a distributed constant circuit filter used in the microwave band and a method of varying the resonance frequency in Patent literature 2 and Non-patent literature 1.
However, the distributed constant circuit filter described above cannot control the bandwidth, although the filter can vary the center frequency.    Patent literature 1: Japanese Patent Application Laid-Open No. 2002-9573 (FIG. 1)    Patent literature 2: Japanese Patent Application Laid-Open No. 2005-253059 (FIG. 1)    Non-patent literature 1: The institute of electronics, information and communication engineers, general conference C-2-37, 2005